Heteromolecular interactions are responsible for a variety of functions. We reported the first observation of reversible tunneling conductivity switching induced by external electric field (EEF) for a proton (H⁺) donor/acceptor bilayer film on Au substrates under ambient conditions at room temperature [1].

Depending on the H⁺-transferred sites, the molecular orbitals of the H⁺-donor and -acceptor molecules are modified. Thus, the H⁺-transfer in the heteromolecular pair is able to control molecular properties and to function as a switch. To achieve this function in molecular systems, catechol-fused tetrathiafulvalene (Cat-TTF) derivatives were introduced, which were designed to exhibit H⁺-electron-correlated properties [2-4]. We fabricated the bilayer films containing Cat-TTF derivatives as a H⁺-donor and imidazole-terminated undecanethiolate self-assembled monolayer (Im-SAM) as a H⁺-acceptor on Au substrates (Fig. 1a), through a two-step immersion procedure. The bilayer film topographies, molecular adsorption states, and physical properties were characterized using several spectroscopic and microscopic methods.

In particular, scanning tunneling microscope (STM) revealed reversible changes in the tunneling conductivity of the bilayer film depending on EEF stimulation. Figure 2a shows a typical STM image of the bilayer film, obtained in constant current (CC) mode. The observed atomically flat terraces and steps imply that the bilayer film is uniform in terms of structurally and quantum tunneling properties. Figures 2b–2d show STM images of the bilayer film after EEF stimulation at various stimulation sample-bias $V_{\mathrm{S}\mathrm{S}}$. At negative $V_{\mathrm{S}\mathrm{S}}$ (-1.5 V), the stimulated area surrounded by red corners in Fig. 2b increased in height change ($\Delta Z>0$), implying increase of tunneling conductivity. This applied EEF may transfer H⁺ from the catechol moiety to the imidazole group in the bilayer film (Fig. 1b). In contrast, at the stimulation by positive $V_{\mathrm{S}\mathrm{S}}$ (+2.0 V) to a part of high conductivity area, surrounded by light-blue corners in Fig. 2c, the area returned to original height ($\Delta Z\approx 0$). This EEF may transfer H⁺ from the imidazole group back to the catechol moiety (Fig. 1c). Moreover, the area surrounded by red corners in Fig. 2d increased in height again by negative $V_{\mathrm{S}\mathrm{S}}$ (-1.5 V). We confirmed that this height change in the CC mode is corresponding to the conductivity change by a $di''/''dv$ mapping method. The mechanism of reversible conductivity switching is considered to be H⁺-transfer by applied EEF. The ability of H⁺-transfer under EEF was theoretically expected in molecular cluster calculations.

Figure 2e shows the EEF-induced height change $\Delta Z$ against $V_{\mathrm{S}\mathrm{S}}$. These results clearly demonstrate two important features: the threshold of the EEF response exhibits hysteresis and H/D isotope dependence. The isotope dependence is crucial evidence of the EEF response due to H⁺-transfer. In addition, the hysteresis in the EEF response indicates that the bilayer film can function as a molecular memory device driven by the H⁺-transfer.

References

• [1] H. S. Kato et al., Nano. Lett. 25, 11116 (2025).
• [2] T. Isono et al., Nat. Commun. 4, 1344 (2013).
• [3] A. Ueda et al., J. Am. Chem. Soc. 136, 12184 (2014).
• [4] H. Mori et al., Chem. Commun. 22, 5668 (2022).

Authors

• H. S. Kato^a, R. Muneyasu^a, T. Fujino^b, A. Ueda^c, Y. Kanematsu^d, M. Tachikawa^e, J. Yoshinobu, and H. Mori
• ^aOsaka University
• ^bYokohama National University
• ^cKumamoto University
• ^dHiroshima University
• ^eYokohama City University